Non-volatile memories usage is widely spreading as they allow reduction of the power consumption of the memory chips, due to the fact that no data refreshing is required. Flash memory is currently being used in a wide variety of devices but suffers from a limited endurance and slow writing times.
Different types of memories are now being intensively developed to overcome these limitations. Among them, Phase Change Random Access Memories (PCRAM) and Magnetic Random Access Memories (MRAM) have been identified by the International Technology Roadmap for Semiconductors (ITRS) as the most adequate for Flash memory replacement.
Typical MRAM structure is based on magnetic tunnel junctions using ferromagnetic materials separated by a thin insulator barrier through which electrons will flow by tunnel effect. One of the ferromagnetic material has its magnetization pinned (so-called pinned layer) while the second ferromagnetic layer is set so that its magnetization can be switched (free layer) from a direction parallel to the direction of the magnetization of the pinned layer (labeled P state) to a direction anti-parallel to the direction of the magnetization of the pinned layer (labeled AP state). The resistance of the AP state is higher than the resistance of the P state, allowing the system to store data as “1” for the high resistance state and “0” for the low resistance state.
Conventional MRAM require a magnetic field to be generated in order to write data (i.e. switch the magnetization of the free layer) and suffer from a lack of scalability due to the current required to generate a high enough magnetic field at small dimensions. It has been demonstrated for some time that passing polarized currents through a magnetic layer can reverse its magnetization, a phenomenon known as spin transfer torque. This effect is the basis of spin torque transfer MRAM (STT-MRAM) and allows high scalability of the storage devices as the current required to write data decreases with the size of the MRAM cell. Typical STT-MRAM require a current density of about 106 A/cm2 to write data and further reduction of this current density is a major topic of research as it would allow the development of low-power consumption devices. However, writing current densities for pure spin-torque effect seem to be approaching a limit.